1. Field of the Invention
The present invention relates in general to monitors, and more particularly, to a method and apparatus for providing adaptive convergence adjustment for a multi-frequency scanning monitor.
2. Description of the Related Art Cathode ray tubes ("CRT") are typically employed in electronic display systems, such as a television receiver or a display apparatus including a CRT numerical display. Each color CRT consists of three electron guns (a red, a green, and a blue electron gun) and a phosphor screen that is located inside an evacuated glass envelope. Each electron gun generates a beam of electrons that is accelerated towards the screen by a positive anode voltage.
Located within the evacuated glass envelope is a color purity magnet that adjusts all three beams at the same time to obtain pure red, green and blue colors. However, refinement is typically needed to aim each beam individually so that the three beams meet at the same point on the screen. Such adjustment for registering all three colors to produce white is called convergence.
The convergence control for individual beam alignment is achieved magnetically, by internal magnetic pole pieces built into the gun assembly. Magnetic flux is coupled into these internal pole pieces, through the glass neck, from an external magnetic assembly called the convergence yoke. The convergence yoke has three magnets, one each for color, namely, red, green, and blue. The adjustment of the permanent magnets is called static convergence, because the magnetic field is steady. Static convergence is adjusted for the center of the picture.
In addition, each coil magnet or electromagnet uses the correction current from the deflection circuits for dynamic convergence. The current can be varied by controls on a separate convergence board. Dynamic convergence is adjusted for the top, bottom, and left and right edges of the picture.
In such conventional convergence control mechanisms, convergence control is typically provided only for signals of a particular frequency, or that of a very narrow frequency range. The placement of the three primary colors is determined according to that particular frequency or that particular narrow range of frequencies. Accordingly, when input signals of a different frequency, or input signals having a frequency that is outside of the very narrow frequency range are encountered, a larger degree of misalignment of the three primary colors occurs. As a result, mismatch of the final color mixture is obtained, for example, a yellowish tint instead of a white color is obtained.
Accordingly, there is a need in the technology for a method and apparatus for providing adaptive convergence adjustment for multi-frequency systems such as a multi-frequency scan monitor.